In U.K. 1,429,494 (U.S. equivalent 3,830,723) a process is disclosed in which high viscosity index lubricating base oils are prepared by catalytic hydrocracking of wax that is obtained in the dewaxing of a residual mineral oil, by separating the hydrocracked product into one or more light fractions and a residual fraction, and by dewaxing the residual fraction to form a lubricating base oil. The dewaxing was carried out using a mixture of solvents. The lubricating base oil obtained in the known process had a viscosity index of up to about 155.
The drawback of the known process resides in the fact that although the viscosity index of the product obtained is excellent, the pour point of the product is not altogether satisfactory for certain applications, such as for use as refrigerator oils. That means that at certain temperatures that are not satisfactorily low, some constituents of the lubricating base oil begin to solidify. These constituents are in particular the unbranched paraffinic molecules.
It has already been acknowledged in the art that the desires as to a low pour point and a high viscosity index are contradictory, and that a balance is to be sought between removing waxy paraffins thereby obtaining a desired low pour point, and retaining branched isoparaffins in the lubricating base oil, which contribute to a good viscosity index. For instance, in EP 225,053 a process is disclosed for the production of a lubricating base oil, referred to therein as lube stock or lubricating oil stock, which has a low pour point and a high viscosity index. This is said to be achieved by a two-step process, in which the intermediate product obtained after a first dewaxing step has a pour point of at least 6.degree. C. above the target pour point, i.e. the pour point of the product obtained after the second dewaxing step. Although this reference alleges that lubricating oil stocks with low pour point and high viscosity index are attainable, it appears from the examples that when a high viscosity index (VI), e.g. above 135, is obtained the pour point is relatively high, e.g. about -6.7.degree. C., whereas when a really low pour point of about -20.degree. C. is obtained the VI has a value of about 100 to 110. It is therefore apparent that the object set in the reference has not quite been achieved.